The present invention relates to a magnetic bearing controller, and more particularly to a magnetic bearing controller which controls a levitated rotating body actively by controlling magnetic force, which is generated by electromagnets by supplying controlled current thereto, whereby the current is controlled by pulse width modulation, and the rotating body is controlled in accordance with the status signals, such as detected displacement sensor signals thereof.
Recently, a magnetic bearing device is becoming to be widely used in the rotary machines in the various fields. The magnetic bearing device levitates and supports a rotating body without contact by magnetic force, which is generated by electromagnet. The advantages of the rotary machines equipped with the magnetic bearings are; abrasion dusts free, maintenance free because lubrication oil is not used, high-speed rotatability, and reduction of noises.
The magnetic bearings are also suitable when these are used in rotary machines, which are disposed in extreme clean atmospheres, such as clean rooms for semiconductor manufacturing. Because the magnetic bearings need no lubrication oil and do not generate abrasion dust, the semiconductor wafers are prevented from being contaminated. Therefore magnetic bearings are advantageous when these are used in clean space, vacuumed space, and so on. Especially in the vacuum space, friction coefficient of usual mechanical bearings becomes extremely large; therefore the magnetic bearings are suitable because there is no problem for the magnetic bearings which support the axes without contact.
FIG. 1 shows a general circuit configuration of a magnetic bearing controller, which controls a levitated rotating body actively by controlling current supply for the electromagnets. The electromagnets are disposed around the rotating body nearby for applying magnetic forces thereto. When the rotating body is levitated and supported by magnetic forces generated by electromagnets without contact thereto, the controlled object of the system is the rotating body, for example, a position thereof. The displacement X of the rotating body comparing the command position X0 is detected by status detector unit, namely displacement sensor unit 11 in this case. The detected position X is compared with the command position X0 in the deviation circuit, and a difference xcex94X between detected position X and command position X0 is inputted to compensator unit 12.
The compensator unit comprises of a control circuit, such as PID (Proportional plus Integral plus Derivative) control circuit, and generates an output signal so as to control that the difference xcex94X between detected position X and command position X0 becomes zero. An output signal of the compensator unit 12 is inputted into signal amplifier unit 13, where the signal is amplified and the amplitude of the signal is limited by a limiter circuit(limiter).
Power amplifier unit 14 generates a controlled current for supplying to electromagnet 15 corresponding with the output signal of the signal amplifier unit 13. The power amplifier unit 14 includes a pulse width modulation circuit for supplying controlled current, which is pulse width modulated thereby into coils of electromagnets 15. The electromagnet 15 generates a magnetic attractive force in accordance with amount of the controlled current. The magnetic attractive force is applied to the controlled object 16, namely the rotating body in this case, and the controlled object is moved so as to be the difference xcex94X is decreased to be zero, namely the detected position X moves to the command position X0. By the above mentioned feedback control, the controlled object 16 (rotating body in this case) is controlled to be stably positioned at the command position X0, even if disturbance force is applied from outwards to the controlled object 15 so as to disturb the position thereof.
In the magnetic bearing controller, a displacement sensor is usually employed as the status detector unit 11 for detecting the status, which is displacement X comparing to the command position X0 in this case. One of typical displacement sensor is an induction type displacement sensor, which has a core of magnetic material with coils wound thereof. According to the inductance type displacement sensor, the displacement of the controlled object which has a magnetic material fixed thereon is measured by the detection of variation of the inductance of the coils thereof.
FIG. 2 shows a detail of signal amplifier unit 13 and a portion of power amplifier unit 14, which includes PWM (Pulse Width Modulation) circuits. The output of compensator unit 12 is inputted to the signal amplifier unit 13 which comprises of an amplitude limiter 25, a deviation circuit 29, a signal amplifier 28, another amplitude limiter 29 and so on. An output of DC signal generator 26 for setting bias DC current is inputted to the deviation circuit 29 for adding the output signal thereto. The power amplifier unit 14 comprises of a PWM circuit having a comparator circuit 31 and a chopping wave generator 32, and a power amplifier circuit 35 which amplifies the output signal of the comparator circuit 31 to the actual current to be supplied to the coils of electromagnets 21.
The output current of the power amplifier 35 is supplied to the coils of electromagnets 21 as a controlled current, thereby controlled magnetic force is generated by electromagnets 21, and applied to the controlled object (rotating body) 22 for controlling the position thereof. The levitated actual position of the controlled object 22 is detected by the status detector, namely the inductance type displacement sensor 23 in this case. The controlled current which is applied to the coils of the electromagnets is detected by a current sensor 36, and lower frequency components of the output signal of the current sensor are returned to the deviation circuit 29 by a feedback loop through a low pass filter 39 for stopping higher frequency components which are corresponding to the frequency components of the PWM signals of the controlled current.
As above mentioned, the signal amplifier unit 13 is comprised of the signal amplifier 28 for amplifying an output signal of compensator unit 12 and the limiter 29 for limiting the amplitude of the amplified signal by the amplifier 28. These circuits are inserted at the front end of the PWM circuit, which comprises of the comparator 31 and the chopping wave generator 32. For driving the coils of electromagnets 21 which are inductive load, relatively large gain, for example 10 through 100 times amplification is required by the signal amplifier 28 which amplifies the output signal of the compensator unit 12 for inputting to the PWM circuit 31 of the power amplifier unit 14.
However, there is a problem that output signal of the signal amplifier unit 13 is deformed to be a rectangular shaped waveform from input sine shaped waveform by passing through the deviation circuit 27, the signal amplifier 28, and the amplitude limiter 29. FIGS. 3A through 3C show the deformation of waveform and frequency spectrum of the deformed waveform. FIG. 3A shows an input signal of sine waveform of 1 kHz, which is inputted to the signal amplifier unit 13. FIG. 3B shows a waveform of controlled current of 1 kHz corresponding to FIG. 3A which flows in the coils of electromagnets showing a rectangular shape. The reason of such deformation of the waveform is estimated that a phase difference is generated in the deviation circuit 27 between input signal and feedback signal, and that the output signal of the deviation circuit is amplified in the signal amplifier 28 including phase differences therebetween and saturated therein by the power supply voltage (xc2x115 V). Therefore, the input signal of the PWM circuit 31 becomes rectangular shaped waveform from original sine shaped waveform by passing through the signal amplifier unit 13.
Accordingly, the controlled current flowing in the coils of electromagnets 21 includes n times harmonic components of fundamental frequency of input sine wave, and the frequency spectrum distribution is shown in FIG. 3C. As shown in FIG. 3C, n times harmonic components are generated from fundamental frequency of input sine wave of 1 kHz. The frequency area where the harmonic components are distributed exceeds more than 10 kHz as shown by circle in FIG. 3C. Further, frequency spectrum distributions over 100 kHz are caused by the PWM circuit 31 for generating pulse width modulated controlled currents, which are modulated by for example 90 kHz chopping frequency.
FIG. 4A shows an input signal of sine waveform of 500 Hz, which is inputted to the signal amplifier unit 13. FIG. 4B shows a waveform of controlled current of 500 Hz corresponding to FIG. 4A which flows in the coils of electromagnets showing a rectangular shape. FIG. 4C shows frequency spectrum distribution of the controlled current corresponding to 500 Hz of FIG. 4B. In the same way as 1 kHz input signal form, sine wave input signal is deformed to be rectangular shaped waveform by passing through the signal amplifier unit 13, and the controlled current is modulated in accordance with the deformed waveform, which flows in the coils of electromagnets. The frequency spectrum distribution includes many n times harmonic components in higher frequency area exceeding more than 10 kHz. It is a problem that harmonic components are distributing nearby 10 kHz frequency area, which is shown by a circle in FIG. 4C.
FIG. 5 shows frequency areas, which are used, in the magnetic bearing controller. Area {circle around (1)} is a frequency area of less than several kHz which is used by the detected signals of displacement sensor, and original controlled currents in the coils of electromagnets. Namely, area {circle around (1)} is used for controlling the rotating body which is detected by the displacement sensor, and controlled by magnetic forces without contact by controlled currents in the coils of electromagnets.
Area {circle around (2)} is a frequency area which is used by the inductance type displacement sensor. The inductance type displacement sensor uses an amplitude modulated signal of, for example, 10 kHz frequency as a fundamental frequency. In accordance with an amount of variation of inductance, the amplitude of the fundamental frequency wave is modulated by the amplitude modulation, and then the position of the rotating body is detected by the change of inductance, namely detecting the amplitude of the modulated signal thereof. Therefore, the frequency spectrums are distributed nearby 10 kHz by the operation of induction type displacement sensor.
Area {circle around (3)} is a frequency area which is used by the PWM circuit 31 for generating controlled current which is supplied to the coils of electromagnets. The PWM circuit 31 uses 90 kHz chopping wave as a fundamental frequency, then the PWM circuit 31 generates the fundamental frequency component and harmonics frequency components by the PWM waveform which are distributed widely over 90 kHz.
FIG. 6 shows a frequency spectrum distribution of more than 1 kHz on which the frequency spectrum distribution that is shown in FIG. 3C or FIG. 4C is superimposed. The inductance type displacement sensor uses nearby 10 kHz frequency area for detecting the displacement of the rotating body by the amplitude modulation of fundamental 10 kHz frequency. However, the frequency area that is used by the inductance type displacement sensor is superimposed by n times harmonics components of the controlled current in the coils of electromagnets. The superimposed signals of two kinds of frequency areas cause the displacement sensor output signal to be deformed or disturbed by harmonics noises of controlled current, and it causes to injure the controllability of the magnetic bearing seriously. Therefore, in a serious case, the controller cannot control the rotating body to be levitated.
It is therefore an object of the present invention to provide a magnetic bearing controller which is able to levitate the controlled object stably by using controlled magnetic forces, even if the status detector unit employs relatively low fundamental frequency for detecting the status of the controlled object.
According to the present invention, there is provided a magnetic bearing controller for supplying a controlled current to an electromagnet for levitating a rotating body at a predetermined position, the controller comprising: an electromagnet for generating magnetic force by the controlled current; a power amplifier unit for supplying the controlled current to the electromagnet, the controlled current being pulse width modulated; a signal amplifier unit for amplifying signal before inputting to the power amplifier; a status detector unit for detecting a status of the rotating body, the rotating body being levitated by magnetic force which is generated by the electromagnet according to the controlled current; and an eliminator unit for eliminating frequency components of frequency area which is used by the status detector unit, the eliminator unit being inserted between the signal amplifier unit and the power amplifier unit.
Accordingly, as an eliminator is inserted between the signal amplifier unit and the power amplifier unit for eliminating frequency components of frequency area which is used by the status detector unit, the harmonics components which are generated by the signal amplifier unit, are eliminated by the eliminator. Then the input signal to the power amplifier unit does not contain the frequency components of frequency area which is used by the status detector unit, and the controlled current which is supplied to the electromagnet by the power amplifier unit also does not contain the frequency components of frequency area which is used by the status detector unit. Therefore, the status detector unit is prevented from malfunction, which is caused by harmonics components generated by the signal amplifier unit, and the magnetic bearing controller is able to operate stably and rapidly for controlling the controlled object.
The power amplifier unit is provided with a pulse width modulation circuit which comprises of comparator for comparing an input signal with chopper wave signal, and the eliminator is connected at the front end of said comparator. Accordingly, harmonics components of the frequency area, which is used by the status detector unit, are eliminated at the front end of the pulse width modulation circuit, and the status detector is prevented from being disturbed by harmonics noises which are contained in the controlled current.
The status detector is an induction type displacement sensor. Accordingly, an induction type displacement sensor is able to be adopted as the status detector, which is easily available and generally or widely used in the industry. Furthermore, the fundamental frequency of the sensor is able to be lowered for suitably using in such as canned encapsulated type magnetic bearing.
The eliminator is a band eliminator filter. Accordingly, the harmonics components of the frequency area which is used by the status detector unit, is effectively eliminated easily.